The present invention relates to machines for grinding ophthalmic glasses and more particularly to the control of the wear of the grinding wheel of such machines.
As is known, a machine for grinding or machining the contour of ophthalmic glasses usually comprises a first fixed and horizontal shaft on which a U-shaped carriage is mounted by an intermediate part of the carriage so as to be slidable in a reciprocating manner, a second shaft parallel to the first shaft and carrying a rotary grinding wheel, and a third shaft parallel to the first and second shafts and rotatably mounted on the ends of the branches of the carriage, divided into two parts between the branches so as to grip between the two parts a glass blank to be ground in accordance with a template, which is carried on an extension of the third shaft outside the branches and bears against a tracer member.
According to French patent No. 2,481,635 in the name of the applicant, a tracer member is mounted to be adjustable in a direction perpendicular to the shaft of the grinding wheel on a rod of a shifting device and connected to a position sensor connected to an electronic device controlling said shifting device.
This electronic device may be associated with a microprocessor which stores in a memory characteristics of a number of glass rims.
Originally, when the grinding machine is brought into service, the distance between the shaft of the grinding wheel and the shaft of a given template is a known value, but this value diminishes with the progressive wear of the grinding wheel, while the diameter of the ground glass on the contrary increases owing to the fact that the shifting device imposes the theoretical distance between the shafts in the course of the rotation of the blank. The operator is consequently obliged to proceed to manual interventions for the purpose of measuring this wear and recalibrating the grinding machine by setting the position of the tracer member.
Document EP-A-0 235 021 describes and shows a machine for grinding ophthalmic glasses which, in one of the embodiments represented in the figures, comprises an adjustable tracer member for bearing against a template connected to a position sensor and mounted on a mechanical shifting device for compensating for the wear of the grinding wheel. In order to permit evaluating the wear of the grinding wheel and to automatically adjust the tracer member for compensating for the wear by modifying the distance between the shafts which respectively carry the grinding wheel and the template, a standard disk is employed and mounted on the shaft instead of a glass blank to be ground, then the distance between the two shafts is measured by bringing the standard disk in contact with the grinding wheel. Apart from these means for indirectly measuring the outside diameter of the grinding wheel, the machine comprises arithmetic means for calculating the amount of wear of the grinding wheel as a function of a measurement effected by the measuring means, adjusting means for adjusting the distance between the shafts driving the grinding wheel and the glass, and control means for controlling the adjusting means in accordance with the amount of wear and thereby achieving a correction of the wear. The adjusting method proposed in this document is, however, difficult and time consuming in use, since it requires putting a standard disk in the place of a glass blank each time it is desired to effect a correcting operation.